In this moralistic fantasy, the third in the Magic Shop series that began with Jeremy Thatcher, Dragon Hatcher, Charlie--passionate environmentalist and compulsive liar--stumbles on Mr. Elives's mysterious shop. He comes away carrying a wisecracking human skull that forces him, and those around him, to tell the truth. The skull helps Charlie sort out his personal life and save some swamp creatures from extinction, telling bad jokes all the while (e.g., ""I couldn't tell a lie if my life depended on it,"" the skull explains. ""If my life depended on it! Oh, god--I kill me!"") Two subplots emphasize the importance of honesty on the one hand (Charlie's beloved Uncle Bennie comes out of the closet) and tact on the other (Charlie's friend Gilbert, bald from chemotherapy, is deeply hurt by truthful comments about his appearance). An unusual resolution to the latter problem finds Charlie and all his friends, girlfriend included, shaving their heads. Magic fans may wish for more of the intriguing shop and its talking rats and less of the skull's simple curse. An excursion into the story of Hamlet is meandering and poorly integrated (the skull, alas, was once poor Yorick). Though the tale itself is lackluster, Coville does keep readers engaged with tight plotting and an otherworldly climax. Black-and-white illustrations not seen by PW. Ages 8-12. (Sept.)